The present invention relates to a development apparatus for developing a latent electrostatic image formed on a photoconductive recording material in a dry type electrophotographic copying machine employing a one-component type developer.
A conventional an apparatus for developing a latent electrostatic image formed on a photoconductor by use of a one-component type developer is disclosed in Japanese laid-open patent application Sho-47-13088 and Japanese patent Sho-51-36070, as well as U.S. Pat. No. 3,731,146.
As shown in FIG. 1, in such development apparatus, a latent electrostatic image formed on a photoconductor drum 1 is developed by a development roller 2 which is rotated in the direction of the arrow. The development roller 2 is made of an elastic conductive material.
The one-component type developer placed in a container 3 adheres to the surface of the development roller 2 and is carried to the photoconductor drum 1. While the developer is carried to the drum 1, the developer is triboelectrically charged by a plurality of triboelectric charging and toner distributing blades 4. The developer exists from under the blades 4 in the form of a thin layer on the surface of the development roller 2. By bringing the thin developer layer into pressure contact with the latent electrostatic image, development is performed. The blades 4 are made of Teflon or Delrin or other appropriate resins.
However, in this development apparatus, since the developer is charged triboelectrically, the development apparatus has the following shortcomings. (1) The developer is not charged speedily; (2) The charging polarity and the potential level of the developer are determined only by the developer and the material of the triboelectric blade so that they cannot be changed readily; and (3) The triboelectric charging blade are worn during use, which results in the triboelectric charging efficiency of the blades being lowered over prolonged use.
Furthermore, in order to allow a negative copy to be made from a positive original or a positive copy to be made from a negative original by use of the conventional development apparatus, two methods can generally be used. One method is to change the polarity of the latent electrostatic image formed on the photoconductor so as to produce the require charge pattern, and the other method is to develop the latent electrostatic image by use of one of two types of developers, each of which is electrically charged to a polarity opposite to that of the other.
In the first method, it is necessary to use two types of photoconductors whose charging polarities are different, or a photoconductor which can be charged to two different polarities. In order to do this, there is a limitation to the kind and the shape of the photoconductor which can be used in this method. Moreover, the polarity of a high voltage generating apparatus for use with a positive or negative corona charger has to be changed, which makes the development apparatus complicated in mechanism and expensive.
In the second method, two types of developers with different polarities have to be used. Thus, in the case of a copying apparatus having only one development apparatus, one developer has to be replaced completely with the other developer whenever the development process is changed. This is greatly time consuming. Further, it is difficult to discharge completely the developer that has been placed in the development apparatus, and when the succeeding developer is mixed with the preceding developer, a background deposition and other troubles are apt to occur. Alternatively a copying machine having two development apparatus can be used and one apparatus would be operated while the other apparatus is unoperated. Providing two development apparatus in one copying machine, however, makes the copying machine over-sized and expensive.